A DNA‐Unwinding Enzyme Induced in Bacteriophage‐T4‐Infected Escherichia coli Cells

Abstract

A single-stranded DNA-dependent ATP .gamma.-phosphohydrolase of MW 56,000 induced after the infection of E. coli cells with bacteriophage T4, probably the ATPase dependent on gene dda of the phage, was isolated. Studies on the enzyme show that with ATP and Mg2+ ions it is capable of dissociating partially double-stranded fd bacteriophage DNA into the single strands and that some 3000-enzyme copies are required to unwind the 6400-nucleotides-long DNA. Unwinding is inhibited by reducing the length of the single-stranded portion of DNA to 2 nucleotides. It can also be inhibited by SH reagents which block the ATPase or by trapping free enzyme molecules in the assay system. Unwinding is initiated near the single-stranded portion of the DNA and is driven by the ATPase. The enzyme apparently unwinds by absorbing to the DNA. Affinity of the enzyme for double-stranded DNA is not detectable by DNA binding assay.